An oil control ring is attached in an annular groove formed on a cylindrical surface of a piston, and is brought into contact with an inner wall of a cylinder to restrict amount of oil that flows into combustion chamber during operation of an engine. An oil ring so called as "two piece type" comprises a space expander and a side rail supported thereon as shown in Japanese Patent Publication Nos. 2-2512 and 3-61860 and U.S. Pat. No. 5,052,698. Such an oil ring of two piece type is disadvantageous in that the spacer expander of the oil control ring is so rigid that during operation of the engine the side rail cannot strictly follow a deformed bore of the cylinder at an elevated temperature, thus resulting in insufficient follow-up property of the side rail in a circumferential direction of the side rail. The insufficient follow-up property of the side rail allows to produce, between the side rail and the inner wall of the cylinder during operation of the engine, radial clearance or gap which increases amount of oil entering a combustion chamber for increase of oil consumption. Therefore, in some cases, the engine cannot generate full power due to increase of oil amount entering the combustion chamber or may produce trouble in operation. In particular, oil control rings shown in Japanese Patent Publication No. 3-61860 and U.S. Pat. No. 5052698, the spacer expander indicates very little deflection during reciprocal motion of the piston because of high rigidity of the space expander so that an inner periphery of the side rail always closely contacts an upper surface of the annular groove formed on the piston. Accordingly, no radial passage is formed between the side rail and the upper surface of the annular groove during the upward stroke of the piston, and the oil scraped by the side rail cannot be drained inside the side rail, thus resulting in increase of oil consumption.
A typical oil control ring of three piece type mainly applied to current internal combustion engines, comprises a space expander and a pair of side rails positioned on and beneath the space expander. However, it is very difficult to manufacture oil control rings of three piece type in such thin structure as recent high speed and light weight engines require.
Accordingly, prior art oil control rings indicate various defects. Firstly, high rigidity of the spacer expander causes insufficient contact of the side rail with an inner wall of a cylinder along the outer periphery of the side rail due to deformation of the cylinder bore at an elevated temperature. Secondly, the spacer expander does not produce deflection in the sliding direction so that the inner periphery of the side rail is always kept in close contact to an upper surface of the annular groove formed on the cylinder during upward stroke of the piston, and therefore no radial passage can be formed between the side rail and upper surface of the annular groove to drain oil scraped by the side rail. Thirdly, oil control rings cannot be made into thin structure in accordance with requirement of light weight and high speed engines.
Accordingly, an object of the present invention is to provide an oil control ring of two piece type made in thin structure.
Another object of the invention is to provide an oil control ring capable of producing a radial passage between a side rail and an upper surface of an annular groove of a piston during the upward stroke to drain oil scraped by the side rail through the radial passage.
Still another object of the invention is to provide an oil control ring having a side rail which is kept in close contact to an inner wall of a cylinder.
A further object of the invention is to provide an oil control ring which can form a radial passage between a side rail and an upper surface of the annular groove during upward stroke of the piston due to deflection of supports of a spacer expander to drain oil scraped by the side rail into a crank chamber.
Still further object of the invention is to provide an oil control ring operable with reduced oil consumption.